In recent years, highly efficient mechanisms for light emission utilizing photon upconversion, particularly upconversion involving triplet-triplet annihilation (TTA: T-T Annihilation), (TTA-UC), have been attracting attention. Photon upconversion is for converting low-energy light, such as near-infrared light, to high-energy light, such as visible light.
The mechanism for light emission enables excitation of all light-emitting centers, for red, blue, and green, by using a photosensitizer and further, particularly for emission of blue light, enables realization of emission of shorter wavelength light, and hence, the mechanism is expected to be applied to, for example, organic EL displays (PTL 1).
TTA occurs when triplet excitons collide with each other, and thus it is necessary to produce overlaps of the wave functions between acceptor molecules and it is desirable that the distance between acceptor molecules be from 0 nm to 1 nm. Thus, in the field of study of upconversion utilizing TTA, a prevalent method is one in which donors and acceptors are mixed together in a volatile organic solvent to cause energy transfer by utilizing the diffusion of donors and acceptors in the solution. Thus, it is not easy to apply upconversion utilizing TTA to devices such as solar cells and organic EL displays.
On the other hand, there are examples in which donors and acceptors are dispersed in a polymer such as polymethylmethacrylate (PMMA). However, in a polymer, molecules cannot easily diffuse and collide and thus it is difficult to achieve high efficiency.
Recently, further examples have been reported. In one example, donors are dispersed in molecularly assembled acceptors to achieve very highly efficient (quantum efficiency of 30%) light emission in a solution. In another example, highly efficient TTA-UC in a solid is successfully performed by synthesizing nanocrystals of metal organic frameworks (MOF: Metal Organic Frameworks) by using acceptor molecules as building blocks and coating the surface of the crystals with donor molecules. The techniques described above, however, cannot be accomplished by an in-situ vacuum vapor deposition process and thus, because of the limitations on the process, are difficult to apply to EL elements that are produced by layering organic films in a vacuum.
Additionally, PTL 2 discloses a visually homogeneous and transparent light conversion element, wherein organic photosensitizing molecules and organic light-emitting molecules, which are a combination that exhibits a triplet-triplet annihilation process, are dissolved and/or dispersed in an ionic liquid. The light conversion element of PTL 2 is obtained through a process that includes adding a solution in which organic photosensitizing molecules are dissolved in toluene and a solution in which organic light-emitting molecules are dissolved in toluene to an ionic liquid, performing stirring for homogenization, and thereafter removing the toluene.
The light conversion element of PTL 2 is able to convert input light into high-intensity light by using upconversion involving TTA and can be applied to, for example, solar cells, and the like.